The manufacture of plastics typically involves extruding the raw, melted plastic material from an extruder and then forming the raw plastic into substantially spherical particles. The plastic particles are entrained in a cooling transport fluid, which is typically water, to form a slurry. The slurry then carries the particulates to other locations in the plastics manufacturing plant for further processing, including but not limited to: removal of undesirable materials, including but not limited to agglomerates; dewatering, that is, removal of some portion of the transport fluid from the slurry; and packaging for shipment to manufacturers of plastic products. Agglomerates, generally speaking, are clumps of particulates. In the plastics manufacturing industry, such agglomerates form as the hot plastic beads clump together within the transport fluid before having a sufficient time to cool. The hot outer surfaces of the plastic beads can in some circumstances melt or otherwise join together to form undesired clumps of raw plastic.
In the prior art devices used to remove agglomerates, one example of which is shown in FIG. 5, the slurry was brought into the apparatus A through a piping system that provided a flow in the apparatus A directly from an overhead pipe B. The slurry flow passed over a screen C containing holes or apertures sized to allow the desired plastic particles to pass through while not allowing agglomerations of the particulates to do so. Typically the velocity of the slurry over the screen C was the same as that of the slurry in the transport pipes to the screen. In many such prior art agglomerate removal operations the agglomerate removal unit is located at some extended transport distance from the extruder and are often found at an elevated location relative to the extruder, such as on an upper floor of the plastics operation facility. This extended distance between the extruder and the agglomerate removal unit is provided to allow the slurry the opportunity to cool. Thus, the slurry may be transported some distance through piping including multiple turns as well as to a higher elevation than the extruder that forms it.
In many if not most uses, such an aforementioned agglomerate removal unit screen C took the form of a grid comprising a plurality of spaced apart rods or ribs set at an angle. As the slurry flowed over the grid, the fluid and the plastic beads would pass through the screening apertures while the agglomerations would be trapped on the upper surface of the grid and washed off by the flow of the transport fluid into a discharge chute D and from there into a waste collector. Often the entrance to the chute D would be blocked by a pivotable door or gate E, against which the agglomerations would accumulate until a sufficient amount of material was accumulated to cause a triggering of a photoelectric or other sensor and the subsequent activation of a pneumatic cylinder to open the door or gate E, allowing the agglomerations to be moved into the chute D by the force of the flow of the slurry and gravity. The slurry less the agglomerates then travels onto a dewatering unit F for separation of the particulates from the transport fluid. The dewatering unit F included a central flanged tower G surrounded by a flanged screen H sized to allow fluid through but not particles. The screened slurry would thus pass into the unit F, the particles entrained in the transport fluid bounce between the flanged tower G and the screen, with the fluid flowing through the screen H and out through a transport fluid exit pipe I. The particles and some fluid will exit the apparatus A through a discharge pipe J and will be taken for further processing elsewhere. In many instances such processing will include drying within a dryer.
The prior art devices manifest several deficiencies. First, they ignored or failed to control the flow rate of the slurry prior to the agglomerate removal unit. In other words, they failed to provide an optimum process flow velocity to the agglomerate removal unit and thus failed to optimize the operation of the agglomerate removal unit. Flow rate variables such as gravity, pipe size, pump size, and particulate type can affect the flow rate and thus the performance of equipment operations downstream of where the slurry is formed. Such variables can cause the flow rate to vary widely, such as, by way of example only, from about 3 feet per second (37.5 centimeters per second) to over 15 feet per second (187.5 centimeters per second). Velocity variances as found in the prior art devices can have several problematic effects.
First, a low slurry flow rate, which can arise from using oversized piping, can magnify the effects of gravity on the continuity of the process flow. That is, where there is a low slurry flow velocity, the pressure drop associated with piping elbows or other piping components is minimized or negligible. Where a particular installation includes large vertical drops in the slurry piping, gravity can accelerate the flow faster than the slurry is being pumped, in turn leading to an uneven or non-continuous flow of slurry into the agglomerate removal unit.
Second, a low flow velocity can facilitate the separation of the particulates or transported materials from the transport fluid in certain circumstances. This separation can occur where the effects of the relative specific gravities of the transport fluid and the transported particulates overcomes those of the flow velocity. Stated otherwise, where the flow velocity gets too low, an additional effect of gravity is that the transported particulates can rise or sink relative to the molecules of the transport fluid. This effect can be avoided by maintaining a defined minimum flow velocity.
Third, a flow velocity that is too high can create problems of another kind. High flow velocities can lead to plugging and leakage problems at the agglomerate removal unit. In addition, a high flow velocity implies a high pumping capacity, which in some cases can be greater than that actually needed and thus implies economic inefficiencies in the plant operation.
Another deficiency of prior art apparatus is that they require all of the transport fluid to pass through the agglomerate removal screen or grid. This high fluid flow rate can cause the screening apertures to become plugged with particulates. In other words, the high volume flow rate can force the particulates to become wedged within the screening apertures.
A third deficiency is that prior art designs utilized a vertical fluid flow onto the angularly disposed agglomerate screen. As seen in FIG. 5, often the flow of the slurry over the screen C would be directly downwardly from the inlet pipe at high velocity. This design can create at least two separate problems. One of these problems is that the vertical slurry drop allowed gravity to accelerate the slurry to a high velocity. The high velocity in turn can cause the agglomerates to impact the screening grid with sufficient force to become wedged in the screening apertures. This problem is exacerbated where the transported particulates are warm plastic beads because once wedged in the screening apertures they will cool further and harden, thus becoming very difficult to remove. As other agglomerates impact the already wedged particles they may stick to each other. This process can continue until the screen is substantially or totally obstructed and the slurry can no longer pass through it.
A second problem with the prior art designs is that the angle of the impact between the fluid flow and the planar screen surface was conducive to screen plugging, particularly where the agglomerates were not elastic as when they comprised soft, warm plastic beads.
Yet another deficiency with prior art designs is that they typically have an inlet to the agglomerate removal unit that is a simple outlet of a pressurized transport pipe. As the slurry exits the transport pipe, it flares or expands in cross sectional area, leading to a turbulent flow profile. In turn, this flow profile made containment of the slurry flow within the desired zone or area difficult.
Another design deficiency in prior art agglomerate removal devices arose from their use of an agglomerate gate such as the gate E shown in FIG. 5. These gates retain the agglomerates removed from the slurry until a certain volume or weight of agglomerates has been accumulated. When the required amount of agglomerates has been reached, the gate will open and the agglomerates will be discharged out a discharge chute. Often gates will fail to seal properly when closing. This failure to seal can be caused by agglomerates being caught in the gate. An improperly sealed gate can leak transport fluid and any particulates that may have accumulated, leading to the loss of product and a potential employee hazard due to the leakage of potentially scalding transport fluid beyond the agglomerate removal chute.
In addition, where the particulate material is plastic, the agglomerates can themselves stick to each other and become a larger, cooling mass. Finally, the use of such gates requires expensive electrical and mechanical components to open and close the gate at the desired times.
A final disadvantage of the prior art designs is the danger they posed to a worker who was required to clean them. As previously noted, the prior art devices often became plugged. Cleaning had to take place during operation, meaning that the worker was exposed to the danger posed by splashing high temperature transport fluids. For example, where the transport fluid is water and the particulates comprise raw plastic beads, the temperature of the transport fluid can become high enough to scald a worker. Thus, scalding protective clothing is required. The prior art designs exacerbated the problem by allowing an uncontrolled splashing of the transport fluid.
It would be desirable to have a new and improved agglomerate and dewatering unit that was not subject to the foregoing disadvantages.